1. Field of the Invention
The present invention relates to a digital image forming apparatus such as a digital printer or a digital copying machine which expresses gradation by modulating light intensity.
2. Description of the Prior Art
In a digital image forming apparatus such as a digital printer or a digital copying machine which expresses gradation by modulating light intensity, a photoconductor is exposed in raster scan with a laser beam at an intensity modulated according to density level of image data of a document detected by an image reader. In the raster scan, a laser beam exposes the photoconductor during a time in correspondence to a length of one dot. Thus, an electrostatic latent image is formed on the photoconductor by forming each dot with a laser beam at an optical intensity in correspondence to density level data.
Unfortunately, the density level of the image data of a document is not proportional to an output density level of an image reproduced on a paper due to various conditions such as photoelectric characteristic of the photoconductor, toner characteristic and environment factors such as humidity and temperature. That is, the density level of image data of a document changes nonlinearly with the output density level of a reproduced image. This is called as gamma characteristic (or gradation characteristic) which is a large factor which deteriorates fidelity of a reproduced image especially for a half-tone image.
Then, the density level of image data of a document are corrected beforehand in order to correct the gradation characteristic, and exposure is performed according to the corrected gradation characteristic. Thus, for example, a linear relation between the document density and the output density can be realized. This is called as gradation correction (or gamma correction). In many cases, a linear characteristic is desired.
As mentioned above, the nonlinearity depends on many factors such as photoelectric characteristic, toner characteristic and environment factors. The nonlinearity changes for example with the sensitivity of the photoconductor or with a charge amount of a development material. If the nonlinearity of gradation characteristic is large, an image data has to be corrected by a large amount, and the correction may enhance a characteristic change, and the precision of the correction decreases. Then, the stability of gradation is liable to be deteriorated. Therefore, it is desirable that the nonlinearity of the gradation characteristic is small.